A basic purpose of selecting a cell is to receive a service from a base station. If signal strength or quality between a user equipment (UE) and a Node B is lowered due to movement of the UE, the UE reselects another cell in order to maintain data transmission quality. Hereinafter, physical signal characteristics related to the strength of a signal or the ratio of noise/interference of a signal will be referred to as signal characteristics.
A cell reselection method may be divided according to a radio access technology (RAT) of a cell participating in reselecting a cell and to a frequency characteristic as follows:                intra-frequency cell reselection in which a UE reselects a cell which uses the same RAT and the same center frequency as a serving cell;        inter-frequency cell reselection in which a UE reselects a cell which uses the same RAT as a serving cell and a different center frequency from the serving cell; and        inter-RAT cell reselection in which a UE reselects a cell which uses a different RAT from a serving cell.        
Referring to FIG. 1, if a UE is powered on, the UE starts operation in an idle mode. The UE automatically or manually selects a public land mobile network (PLMN) which is to be served and a RAT for communication (step S110). Information about the PLMN and RAT may be selected by the user of the UE or may be stored in a universal subscribed identity module (USIM).
The UE performs an initial cell selection process for selecting a cell having the greatest signal strength or quality value among cells having signal strength or quality values greater than a reference value (step S120). The reference value refers to a value defined in a system in order to guarantee quality of a physical signal during data transmission and reception. Accordingly, the reference value may vary with an applied RAT. The UE receives system information (SI) which is periodically transmitted by the Node B. The system information includes basic and indispensable information necessary when the UE accesses a network. The system information may include a neighbor cell list (NCL) related to cells adjacent to a serving cell. Therefore, the UE should receive all the system information before accessing the Node B and always have the latest system information. If power is on, the UE selects a cell in order to receive the system information in an idle mode.
The UE registers information (e.g., international mobile station identifier (IMSI)) thereof in a network to receive a service (e.g., paging) from the network (step S150). The UE does not always register the information thereof in the network whenever selecting a cell and registers the information when network information (e.g., tracking area identity (TAI)) received from the system information is different from network information known thereto (step S140).
If a signal strength or quality value measured from the Node B of the serving cell is lower than a value measured from a Node B of a neighbor cell, the UE selects another cell providing better signal characteristics than the cell of the Node B. If a priority is defined between frequencies or RATS, the UE selects a cell in consideration of the priority (step S160). This process is called a cell reselection process to discriminate from the initial cell selection of step S120. In a long term evolution (LTE) system, reference symbol received power (RSRP), reference symbol received quality (RSRQ), and received signal strength indicator (RSSI) are being discussed as a signal measurement value.
An operation and state performed when the UE in idle mode reselects a cell in step S160 will now be described in detail.
Basically, the UE can receive a normal service in a suitable cell. When the UE has not selected a suitable cell, the UE can receive only a limited service such as an emergency call from an acceptable cell. A description of the terms is as follows.
A. Acceptable cell                not barred: whether a cell is acceptable or not is indicated as one bit through system information.        when an acceptable state is designated, the acceptable cell satisfies a minimum signal characteristic value for selecting a cell (i.e., satisfies a minimum cell reselection criterion).        
B. Suitable cell                satisfies a condition of an acceptable cell.        PLMN registered by a UE or PLMN to which a serving cell belongs.        
To perform an operation for selecting the acceptable cell or suitable cell, a state of a UE may be defined. The state and operation of the UE are as follows.
A. ‘Any Cell Selection’ state: a state for attempting to camp on an acceptable cell.
B. ‘Camped-on Any Cell’ state: a state camped on an acceptable cell and a state for periodically attempting cell reselection for camping on a suitable cell. A limited service such as an emergency call can be received. When camped on a suitable cell, the ‘Camped-on Any Cell’ state transitions to ‘Camped normally’ state. The following operation is performed:                receipt of a paging message        receipt of system information        measurement for cell reselection        cell reselection        periodic search of a suitable cell        
C. ‘Camped normally’ state: a state camped on a suitable cell. If a better suitable cell than a serving cell is found, cell reselection is performed. If another suitable cell is not found even though the serving cell does not satisfy a condition of a suitable cell (e.g., a signal characteristic value of the serving cell is lower than a threshold value), the ‘Camped normally’ state transitions to ‘Any Cell Selection’ state to search an acceptable cell. The following services can be performed:                receipt of a paging message        receipt of system information        measurement for cell reselection        cell reselection        
Being camped on a cell refers that a UE tunes a central frequency of the cell and use a control channel. Therefore, the UE can receive control information such as a paging message and system information from a camped-on cell. The UE can also perform a random access process or can request the eNode B of a radio resource control (RRC) connection.
A basic operation in FIG. 1 for the UE to reselect a cell according to signal characteristics has been described. In a LTE system, the following is considered as a case requiring cell selection:                UE capability        subscriber information        camp load balancing        traffic load balancing        
UE capability refers to select a cell according to a frequency band which can be selected by a UE because the UE may have a limited available frequency band. The UE may be set to select or not to select a specific cell according to subscriber information or operator policy. Camp load balancing refers to select a cell which is being used by a small number of UEs in order to reduce data load generated when UEs in idle-mode are activated in a single cell. Similarly, traffic load balancing serves to change a cell in order to reduce data load generated by activated UEs.
Especially, a LTE system has a strong tendency toward expansion of a frequency band based on an existing universal terrestrial radio access network (UTRAN) for the purpose of installation/maintenance. UEs in the same cell share radio resources for communication, and thus load balancing between cells needs to be achieved in order to efficiently use the radio resources. Accordingly, camp/traffic load balancing is defined in the LTE system as an indispensable requirement.
In order to effectively achieve the above cell selection process in the LTE system, a priority may be defined with respect to frequencies or RATs so that a UE can select a specific frequency or RAT having the priority during cell selection or cell reselection. When a plurality of frequencies/RATs is present, the frequencies/RATs may have different priorities or the same priority. Alternatively, some of the frequencies/RATs may have the same priority. In the LTE system, only a frequency or RAT having a priority is defined to be selected during cell reselection.